Do you think the size of the bike makes the difference?

Descending ability, like any other fine-motor skill, is best improved with practice. I’d find a good descent and ride it repeatedly. The advantage to repeating the same descent is getting to know the same corners and straights without taking too many if any chances and slowly improving skills, speed and increasing your confidence on your new bike. You can then transfer those skills to your general riding. That confidence from repeating can also help you to start relaxing more. Anxiety can impair concentration, which can cause problems on descents, especially corners. Practice smoothness, no sudden movements or braking and speed and confidence will follow.

Descending technique is also important and there are lots of web articles on that....or even better that could be a good learning/sharing experience with the BF.

Take it easy, relax, concentrate and go slowly and safely while improving your descending skills.